The present invention relates to a surface light system used for backlighting of liquid crystal displays or the like, and particularly to a so-called tandem type surface light source system.
As the surface light source system, there has been known a so-called tandem type surface light source system allowed to obtain a more uniform luminous intensity over a wider area.
A configuration of a related art tandem type surface light source system will be described below with reference to FIGS. 1A and 1B.
Referring to FIG. 1A, there is shown a surface light source system 1 including a plurality (three in the example shown in the figure) of fluorescent tubes 2a, 2b, and 2c which horizontally extend in the direction perpendicular to the longitudinal direction of the system 1 in such a manner as to be spaced from and in parallel to each other; light guide plates 3a, 3b, and 3c made from a transparent material, which extend along right or left sides (right sides in the example shown in the figure) of the fluorescent tubes 2a, 2b, and 2c; reflectors 4a, 4b, and 4c disposed on the opposed sides (left sides in the example shown in the figure) to the light guide plates 3a, 3b, and 3c in such a manner as to surround the fluorescent tubes 2a, 2b, and 2c; reflecting members 5a, 5b, and 5c disposed under the light guide plates 3a, 3b, and 3c; and a lens sheet 6 disposed over the light guide plates 3a, 3b, and 3c. 
Each of the light guide plates 3a, 3b, and 3c is formed into a shape in cross-section which tapers as a wedge in the direction from the corresponding one of the fluorescent tubes 2a, 2b, and 2c to the opposed side. In the example shown in the figure, the light guide plates 3a, 3b, and 3c are integrated with each other.
According to the surface light source system 1 having the above--configuration, light rays emitted from the fluorescent tubes 2a, 2b, and 2c directly enter the light guide plates 3a, 3b, and 3c or are reflected from the inner surfaces of the reflectors 4a, 4b, and 4c and enter the light guide plates 3a, 3b, and 3c; the light rays, which pass through the light guide plates 3a, 3b, and 3c, are reflected from the reflecting members 5a, 5b, and 5c to direct upwardly through the light guide plates 3a, 3b, and 3c, and are diffused by the lens sheet 6; and the light rays thus diffused by the lens sheet 6 light a liquid crystal display (not shown) from the back surface thereof, for example.
The surface light source system 1, however, has a problem. At the ends and their neighborhoods, on the fluorescent tube sides, of the light guide plates 3a, 3b, and 3c, part of light rays, which have entered the light guide plates 3a, 3b, and 3c from the fluorescent tubes 2a, 2b, and 2c directly or by way of reflection from the inner surfaces of the reflectors 4a, 4b, and 4c, may be directly emerged outside therefrom without total reflection at the top surfaces of the light guide plates 3a, 3b, and 3c. 
As a result, as shown in FIG. 1B, at the ends and their neighborhoods, on the fluorescent tube sides, of the light guide plates 3a, 3b, and 3c, the luminance becomes significantly higher, so that uneven luminance portions appear in the luminance distribution of the surface light source system 1. To be more specific, on the top surface of the lens sheet 6, luminance becomes higher at portions near the fluorescent tubes 2a, 2b, and 2c, with a result that there occurs large differences in luminance between the portions near the fluorescent tubes 2a, 2b, and 2c and other portions. In particular, if the light guide plates 3a, 3b, and 3c are divided from each other, bright lines and shades occur along the division lines, thereby failing to obtain a uniform surface light source.
An object of the present invention is to provide a tandem type surface light source system allowing a uniform luminance over the surface light source.
To achieve the above object, according to a first aspect of the present invention, there is provided a surface light source system including: a plurality of bar-like light sources disposed in parallel to each other; reflecting members, each of which is disposed to surround a right or left side, an upper side, and a lower side of the corresponding one of the light sources; light guide plates made from a transparent material, each of which is disposed along the left or right side of the corresponding one of the light sources, and is formed into a taper shape in cross-section along the traveling direction of light; and reflecting sheets, each of which is disposed on the back surface of the corresponding one of the light guide plates; wherein each of at least the second and later ones of the light guide plates is, at its end portion on the light source side, integrated with a light introducing portion made from a transparent material, the light introducing portion extending from the end portion, on the light source side, of the light guide plate to a position under an end portion of the front side light guide plate adjacent to the light guide plate.
With this configuration, the system functions as a surface light source as follows. Light rays enter the light introducing portions directly or indirectly, that is, by way of reflection from the inner surfaces of the reflecting members, being subjected to total reflection from the top or bottom surfaces of the light introducing portions, and enter the light guide plates. The light rays having entered the light guide plates travel throughout the light guide plates while being repeatedly subjected to total reflection from the top surfaces of the light guide plates and to reflection from the reflection sheets located on the back surfaces of the light guide plates, and are emerged from the top surfaces of the light guide plates.
As described above, the light rays having entered the light introducing portions are not directly emerged from the top surfaces of the light guide plates, and accordingly, it is possible to suppress occurrence of locally high luminous lines at ends and their neighborhoods, on the light source sides, of the light guide plates, which luminous lines have occurred in the related art system. This makes it possible to prevent occurrence of unevenness of luminance and hence to obtain a uniform luminance over the surfaces of the light guide plates.
In this system, each of at least the second and later ones of the reflecting sheets may extend along the back surface of the corresponding one of the light introducing portions. With this configuration, when the light rays, which have been emitted from the light sources and entered the light introducing portions, are emerged downwardly therefrom without total reflection from the bottom surfaces of the light introducing portions, the light rays thus emerged downwardly are returned in the light introducing portions by the reflecting sheets.
In the system, an end plane, on the light source side, of each of the light introducing portions may be formed with its upper end tilted in the direction of the light guide plate. With this configuration, since light rays having been emitted from the light sources enter the light introducing portions while being refracted downwardly, the light rays reflected from the bottom surfaces of the light introducing portions are made incident on the top surfaces of the light guide plates at small incident angles, to be thereby less subjected to total reflection from the top surfaces of the light guide plates. This is effective to prevent occurrence of shades at boundaries between the light guide plates.
In the system, a top surface or bottom surface of each of the light introducing portions may be subjected to a coarsening treatment. With this configuration, since light rays, which have been emitted from the light sources and entered the light introducing portions, are irregularly reflected from the coarsened top or bottom surfaces of the light introducing portions, the light rays thus irregularly reflected are less subjected to total reflection from the top surfaces of the light guide plates. This is effective to prevent occurrence of shades at boundaries between the light guide plates.
In the system, each of the light introducing portions may be formed into a taper shape in vertical cross-section. With this configuration, since light rays, which have been emitted from the light sources, entered the light introducing portions, and reflected from the bottom surfaces of the light introducing portions, are made incident to the top surfaces of the light guide plates at small incident angles, the light rays are less subjected to total reflection from the top surfaces of the light guide plates. As a result, it is possible to prevent occurrence of shades at boundaries between the light guide plates.
In the system, a space, which is formed between an end plane, on the light source side, of each of the light introducing portions and the corresponding one of the reflecting members in such a manner as to surround the corresponding one of the light sources, may be filled with a liquid transparent material. With this configuration, since light rays having been emitted from the light sources enter the light introducing portions via the liquid transparent material, it is possible to suppress total reflection from an end plane, on the light source side, of each of the light introducing portions, and hence to improve the incident efficiency from the light introducing portions to the light guide plates.
In the system, a diffusion portion having a light diffusivity different from that of the light guide plates may be disposed on top surfaces of the light guide plates. With this configuration, it is possible to realize a more uniform surface light source.
In the system, a light modulation portion having a dot pattern for reflecting light on the light guide plate side may be disposed between the light guide plates and the diffusion portion, the dot pattern being preferably formed in such a manner that a dot pattern density is changed in the direction apart from each of the light sources. With this configuration, light rays emerged from the light guide plates are irregularly reflected from the dot pattern, and the light rays reflected from the bottom surfaces of the light guide plates are reflected from the light diffusion plate and further reflected from the dot pattern. Accordingly, it is possible to obtain a more uniform light distribution on the top surface of the surface light source.
In particular, the dot pattern may be configured such that the density of the reflection dot pattern becomes higher at a portion closer to the light source and becomes gradually lower at a portion farther apart from the light source. With this configuration, since a portion having a high light quantity is eliminated, it is possible to effectively prevent occurrence of luminous lines.
In the system, the light modulation portion may be formed by printing a light modulation material on the back surface of a diffusion plate constituting the diffusion portion. With this configuration, the light modulation portion can be formed by relatively simple means.
In the system, the light guide plates may be integrated with each other, and each of the light introducing portions may be formed separately from the corresponding one of the light guide plates and be optically connected to an end plane, on the light source side, of the light guide plate. With this configuration, since the separate light introducing portions can be easily, optically connected to the integral type light guide plates of the related art surface light source system, it is possible to reduce the production cost of the system.
In the system, each of the light introducing portions may be optically connected to an end plane, on the light source side, of the corresponding one of the light guide plates by means of a liquid transparent material. With this configuration, since light rays having been emitted from the light sources enter the light introducing portions via the liquid transparent material, it is possible to suppress total reflection from an end plane, on the light source side, of each of the light introducing portions.
In the system, a space, which is formed between an end plane, on the light source side, of each of the light guide plates and the corresponding one of the reflecting members in such a manner as to surround the corresponding one of the light sources, may be filled with the liquid transparent material for forming each of the light introducing portions. With this configuration, it is possible to easily provide the light introducing portions on the related art surface light source system by filling a space between an end plane, on the light source side, of each light introducing portion and the corresponding reflecting member with the liquid transparent material, and hence to reduce the production cost of the system.
In the system, each of the light guide plates may be formed separately from the corresponding one of the light sources; each of the light introducing portions be formed integrally with the corresponding one of the light guide plates; and the light guide plates be optically connected to each other. With this configuration, since each light introducing portion is formed integrally with the corresponding light guide plate, it is possible to reduce the number of parts and production steps and further facilitate the assembling step, and hence to reduce the production cost of the system.
In the system, a diffusion plate having a light diffusivity different from that of the light guide plates may be disposed on top surfaces of the light guide plates, and the light guide plates be fixedly held by the diffusion plate to be thereby integrated with each other. With this configuration, the light guide plates are connected to each other by the diffusion plate, and since the diffusion plate has a light diffusivity different from that of the light guide plates, it is possible to realize a more uniform surface light source.
In the system, a shielding member may be disposed at a boundary plane between a leading end plane of each of the light guide plates and the next one of the light guide plates. With this configuration, since the incidence of one light guide plate on the next light guide plate is shielded by the shielding member, it is possible to eliminate unevenness of luminance due to leakage of light rays from the next light guide plate.
In the system, the shielding member is tilted at a tilt angle being nearly equal to a critical angle determined by a refractive index of a transparent material for forming the light guide plates and the light introducing portions. With this configuration, the light rays reaching the top surface of the next light guide plate along the shielding member are less subjected to total reflection, it is possible to suppress occurrence of shades due to the shielding members, and hence to obtain a uniform luminance.
In the system, the light guide plates may be formed in such a manner that leading ends thereof in the traveling direction of light extend to the same position. With this configuration, since the boundaries between the light guide plates are not exposed from the top surfaces of the light guide plates, it is possible to eliminate occurrence of luminous lines and shades due to the boundaries.
In the system, each of the light introducing portions may have on its top surface a stepped portion for receiving a leading end of the front side light guide plate, and the stepped portion be tiled at a tilt angle nearly equal to a critical angle determined by a refractive index of a material for forming the light introducing portions. With this configuration, since light rays reaching the top surface of the next light guide plate along the stepped portion are less subjected to total reflection, it is possible to suppress occurrence of shades due to the shielding members, and hence to obtain a uniform luminance.
In this way, according to the first aspect of the present invention, since the light introducing portions are provided on the end planes, on the light source sides, of the second and later light guide plates, it is possible to suppress light rays from the light sources from being directly emerged upwardly from the light guide plates. This makes it possible to suppress occurrence of high luminous lines at ends and their neighborhoods, on the light source sides, of the light guide plates, and hence to obtain a uniform luminance over the surfaces of the light guide plates.
To achieve the above object, according to a second aspect of the present invention, there is provided a surface light source system including: a plurality of bar-like light sources disposed in parallel to each other; reflecting members, each of which is disposed to surround a right or left side, an upper side, and a lower side of the corresponding one of the light sources; light guide plates made from a transparent material, each of which is disposed along the left or right side of the corresponding one of the light sources, and is formed into a taper shape in cross-section along the traveling direction of light; and reflecting sheets, each of which is disposed on the back surface of the corresponding one of the light guide plates; wherein a diffusion portion having a light diffusivity larger than that of the interiors of the light guide plates is provided on the light guide plates.
With this configuration, the system functions as a surface light source as follows. Light rays enter the diffusion portion directly or indirectly, that is, by way of reflection from the inner surfaces of the reflecting members, being irregularly reflected from the diffusion portion, and enter the light guide plates. The light rays having entered the light guide plates travel throughout the light guide plates while being repeatedly subjected to total reflection from the top surfaces of the light guide plates and to reflection from the reflection sheets located on the back surfaces of the light guide plates, and are emerged from the top surfaces of the light guide plates.
Since the light rays having entered the light guide plates are made incident on the diffusion portion disposed thereon, they are not directly emerged from the top surfaces of the light guide plates. As a result, it is possible to suppress occurrence of locally high luminous lines at ends and their neighborhoods, on the light source sides, of the light guide plates, which luminous lines have occurred in the related art system. This makes it possible to prevent occurrence of unevenness of luminance and hence to obtain a uniform luminance over the surfaces of the light guide plates.